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[TowerTalk] Re: [DX] [Fwd: Low yagi question]

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Subject: [TowerTalk] Re: [DX] [Fwd: Low yagi question]
From: (T. A. Russell)
Date: Fri, 4 Jul 1997 07:25:09 -0600
>  I did receive one reply from a fellow who claimed that, the yagi 
>antenna tends to "force" a low take off angle when compared to a 
>at the same hight. If this is so, I should enjoy better dx 
>along with the quieter reception from the yagi...even with the low 
>of 40 feet. Is there any truth to this forced low angle radiation from
>yagi antennas?? 
>Thanks, 73 from AB2CJ

The radiation pattern from any antenna over real earth is the product
of it's free space pattern (linear scale) and the ground reflection
coefficient (also linear scale).  The OLD  ARRL antenna books
carried charts showing ground reflection coefficients for various 
heights above ground with a linear scale.

Since the pattern of a dipole in free space is equal at all elevation
the ground reflection coefficient pattern corresponds to the pattern for
a dipole antenna.  Yagi and Quad antennas obtain gain by refocusing
energy in the desired direction.  A  3 element Yagi pattern is typically 
down 3 dB at +/- 50 degrees in the vertical plane, thus reducing 
higher angle radiation.  Stacked Yagi's (and quads) cutoff at even 
lower angles, leading to the impression that they "force" lower angle
radiation.  In fact, all they are doing is reducing high angle radiation.

GROUND   REFLECTION  coefficients depend SOLELY on HEIGHT
above ground (in wavelengths).  A tribander's pattern will peak at
different angles on the different bands because it's (fixed constant)
height  is different for each band in terms of wavelengths. 

BTW, a tribander at 40 ft will cover all the higher angles that propagate
on the high bands.  Separate higher antennas are needed to maximize
the lower angles when MUF is marginal.  BOTH high and low Yagi's
are needed to cover ALL the angles that the ionosphere will support 
due to pattern nulls in the vertical plane caused by ground reflections.

de  Tom  N4KG

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